Nowadays, in drilling of oil well and gas well, it has been required to drill until deeper stratum, and drill under more sever corrosive environment than before.
In such an environment, a seamless pipe made of high Cr-high Ni alloy having a good strength, corrosion resistance, and high-temperature resistance is used. Especially, a high Cr-high Ni—Mo steel is a material having a high strength showing high corrosion resistance even under hot sweet (CO2) environment and hot sour (H2S) environment.
One method that can be considered for productivity improvement and cost reduction in steel process is to change its casting method from an ingot casting method to a continuous casting method. In the ingot casting method, since a final solidification position of a steel ingot is a gate riser part, segregation portion and cavity portion are gathered to the gate riser part (upper portion of steel ingot). Therefore, in the ingot casting method, even though constant part of the steel ingot after the casting has a good quality, since the segregation portion and the cavity portion are required to be removed from the steel ingot after casting, the productivity improvement and cost reduction are restricted.
A process of manufacturing a seamless pipe from a rectangular billet made by continuous casting includes the steps of manufacturing a round billet from the rectangular billet by rolling or forging, and manufacturing a seamless pipe from the round billet. In the step of manufacturing the seamless pipe from the round billet, the internal quality (for example, intergranular cracking, cavity, segregation and the like) of the round billet to be used affects the quality of inner surface of the seamless pipe to be produced. Therefore, regarding the round billet used for manufacturing a seamless pipe, not only quality control of the outer surface but control of internal quality is also important.
In the continuous casting method, since cooling speed is faster comparing with the ingot casting method, cracking can occur in axial center. Also, in a case where a bending type continuous casting apparatus is used, there is a possibility that cracking occurs in accordance with deformation (unbending) of cast slab in moving from a bending part to a horizontal part of the apparatus.
If the continuous casting is tried to be applied to manufacture a high Cr-high Ni—Mo steel for manufacturing a seamless pipe, (1) segregation of Mo tends to be remained in the cast slab, (2) cavity tends to be remained in the cast slab, and (3) intergranular cracking tends to occur due to a high deformation resistance. Therefore, conventionally, it was difficult to employ the continuous casting method as a casting process of a high Cr-high Ni—Mo steel for manufacturing a seamless pipe.
Relating to manufacturing a seamless pipe of high Cr-high Ni based high alloy, Patent Document 1 discloses limiting contents of Zn and Pb contained in the alloy as impurity, and limiting total rolling ratio of billet (=(cross section before rolling)/(cross section after rolling)) in shaping a rectangular billet into a round billet. Patent Document 1 describes that according to this method, since cracking and flaw are difficult to occur on outer surface of the billet, a seamless pipe of high alloy having an improved outer surface quality can be manufactured.
However, regarding the internal quality of a round billet manufactured from a rectangular billet made by continuous casting of high Cr-high Ni—Mo steel and the inner surface quality of the seamless pipe manufactured from the round billet, any findings for quality improvement have not been obtained.